Traditional membrane switches work in conjunction with a light guide and a light source to produce backlighting for a keypad or control panel. FIG. 1 illustrates a portion of a prior art membrane switch assembly 10 such as the type used in cell phones. Pairs of contacts 12, 14 are positioned at spaced locations on a circuit board 16. A snap dome 20 is positioned over each pair of contacts, with the periphery 22 of each snap dome lying on and in constant contact with the outer contact 12. The middle 24 of each snap dome is positioned over a middle contact 14 and touches the middle contact 14 when the snap dome is depressed. This causes the middle contact 14 to engage the outer contact 12. Although a non-snap dome may be used, a snap dome is often used because it provides tactile feedback to the person who depresses it. The upper surfaces of the domes 20 are adhesively fastened to a flexible carrier sheet 30 which are used to hold the domes 20 in selected positions over the pairs of contacts.
A manually depressable key 32 with an identification marking 34 on its upper surface has a transparent or translucent downward projection 36. When the key 32 is depressed, the projection 36 depresses a location on the carrier sheet 30, which depresses the middle 24 of the dome 20 to cause contacts 12 and 14 to close the switch that corresponds to that key. When the depressing force is no longer applied, the dome, a portion of the carrier sheet 30 and the key 32 revert to their original positions. A light guide 40 is mounted above the carrier sheet and has a hole 42 aligned with the carrier sheet portion that is positioned above a dome. The key projection 36 projects through the hole 42. The light guide carries light from a light source and releases some of the light at the hole 42, to pass through and around the projection to illuminate the key.
FIG. 2 is a sectional view of a switch of the assembly of FIG. 1. The membrane switch includes a spacer 50 and two adhesive layers 52, 54 that hold the carrier sheet 30 so it is raised above the circuit board 16 to allow venting of air beneath the dome, and to hold the carrier sheet in place on the circuit board. Lines 61-63 show the paths of light passing from the light source 64 along the light guide 40. The light guide may be fastened by screws to the circuit board or held down by the device housing. FIG. 2 depicts the dome 20 in its quiescent position. Line 20A shows the dome in a depressed or activated position.
Traditional thin light guides caused light to scattered or absorbed and not reflected upward toward the key. Additionally, light from the light source is typically lost inside the membrane switch since oftentimes the brightest part of the light source shines into layers of the membrane switch instead of into the light guide.